1. Field of the Invention
The present invention relates to a toner that is used for development of an electrostatic latent image in an electrophotographic method and an electrostatic recording method, a developer, a production method and an image forming method.
2. Description of the Related Art
Traditionally, a Carson method is generally used when an image is formed in a copier, laser beam printer or the like. In the conventional image forming method, an electrostatic latent image formed on a latent holding member (photosensitive member) is developed with toners containing colorants, a resultant toner image is transferred onto a transferring body (e.g. recording material), and this is fixed by a heat roller or the like to obtain an image, while the latent holding member is cleaned in preparation for forming an electrostatic latent image again. Dry developers for use in this electrophotographic method and the like are classified broadly into single-component developers using solely toners having colorants and the like blended in binder resins, and two-component developers with carriers mixed with the toners. The one-component developer maybe classified into magnetic one-component type in which a latent image is carried to a development carrier (photosensitive member) by a magnetic force using a magnetic powder, and an image is developed, and the nonmagnetic one-component type in which a latent image is carried to a development carrier by charge transfer from a charging roller or the like without using a magnetic powder, and an image is developed.
Since the late-1980s, size reductions and improvements in functions have been increasingly required in the electrophotograph market under the keyword of digitization, and sophisticated printing and high quality close to that of silver salt photography are desired, especially for full color image quality.
Digitization processing is essential as means for achieving high image quality, and effects of such digitization on image quality include a capability of carrying out complicated image processing at a high speed. Consequently, letters and photographic images can be controlled separately, and thus reproducibility of quality of both the letters and images are significantly improved compared with analog techniques. For a photographic image, in particular, digitization makes it possible to perform tone correction and color correction, and is advantageous with respect to tone characteristics, precision, sharpness, color reproducibility and graininess compared to analog techniques. On the other hand, however, for image output, a latent image created in an optical system should be faithfully developed, and for toners, size reductions are being increasingly promoted, and activities aimed at faithful reproducibility are being vigorously pursued. However, it is difficult to achieve high quality with stability by merely reducing the size of the toner, and improvements in basic characteristics in development, transfer and fixing characteristics become more important.
Particularly in color images, color toners of three or four colors are superimposed to form an image. Therefore, if any one of the toners exhibits characteristics different from initial characteristics or a performance different from those of other color toners in terms of development, transfer and fixing, a reduction in color reproducibility and degradation of image quality such as degradation of graininess and color shading will be caused. How the characteristics of each toner are controlled with stability is important for maintaining stable high quality images equivalent to those at the initial stage even as time passes.
Electrophotographic dry developers that have been used for development of an electrostatic latent image formed on an electrophotographic photosensitive layer generally include a one-component developer comprised of a toner obtained by melt-kneading a binder resin constituted by resins such as polystyrene, a styrene-butadiene copolymer, a copolymer of styrene-acryl based monomers, polyester and polyepoxy with a pigment or dye such as carbon black or phthalocyanine blue as a colorant, and crushing the same, or a two-component developer prepared by mixing a toner with particles of glass bead, iron, nickel, ferrite or the like having an average particle size almost equal to that of the toner or 500 μm or smaller, or such a material covered with various kinds of resins as a carrier. In the two-component developer, since the toner is caused to be frictionally charged by stirring the toner and the carrier, the frictional charge amount of the toner can be appropriately controlled by selecting characteristics of the carrier and stirring conditions, and therefore excellent image quality is achieved with high reliability.
However, the above toner and developer alone cannot achieve adequate characteristics such as storage stability (blocking resistance), transportability, developing characteristics, transformability and charge characteristics. Thus, Japanese Patent Laid-Open Publication No. Hei 4-204750, Japanese Patent Laid-Open Publication No. Hei 6-208241, Japanese Patent Laid-Open Publication No. Hei 7-295293 and Japanese Patent Laid-Open Publication No. Hei 8-160659 propose that an additive such as silica or titanium oxide, or an additive prepared by treating its surface with an organic silane compound to impart a hydrophobic nature thereto, or covering the surface with an inorganic oxide, is externally added for the purpose of improving those characteristics. However, these measures bring about some degree of improvements in storage stability (blocking resistance), transportability, developing characteristics, transformability, charge characteristics, but provide no improvement in fixing characteristics. In addition, Japanese Patent Laid-Open Publication No. Hei 8-190221 proposes the use of abrasive particles with a particle size of 0.1 to 10 μm containing calcium carbonate to prevent contamination of the surface of the photosensitive member. In addition, Japanese Patent Laid-Open Publication No. 2002-287411 proposes a toner for a recycle system such that calcium carbonate particles are deposited on the toner to prevent contamination of the surface of the photosensitive member. However, no improvements in fixing characteristics have been found.
On the other hand, Japanese Patent Laid-Open Publication No. Sho 56-125751, Japanese Patent Laid-Open Publication No. Sho 62-267766 and Japanese Patent Publication No. Hei 7-120086 propose methods in which the volume specific resistance of the carrier is controlled to faithfully reproduce a high quality image, specifically halftones, black solids and letters. In these methods, the resistance is adjusted by the type of carrier coating layer and the coating amount, and a desired volume specific resistance can be achieved to form high quality images in the initial stage, but peeling in the carrier coating layer or the like occurs under stress in a developing device, causing the volume specific resistance to vary significantly. Thus, it is difficult to form high quality images over a long period of time.
On the other hand, Japanese Patent Laid-Open Publication No. Hei 4-40471 proposes a method in which carbon black is added in the carrier coating layer to adjust the volume specific resistance.
According to this method, the volume specific resistance can be prevented from being varied due to peeling in the coating layer, but external additives added in the toner or toner constituent components are deposited on the carrier to cause the volume specific resistance to vary, thus making it difficult to form high quality images over a long period of time as in the case of the carrier described above.
In addition, Japanese Patent Laid-Open Publication No. Hei 9-325513 proposes a toner for electrostatic image developer made by depositing on the surfaces of image developing electrostatic toner particles of at least one type of carbonate fine particles selected from a group consisting of calcium carbonate, barium carbonate, strontium and zinc carbonate having a primary average particle size of 0.01 to 0.5 μm and a specific surface area of 25 m2/g to 200 m2/g for providing good developed images and maintaining high image quality over a long period of time.
On the other hand, hitherto, monochrome images have been formed when forming images on double sides, and in this case, images in a recording material have high quality if the above described toner is used. In the case of forming color images on double sides, however, since an image is transferred to the first face of the recording material and also to the second face corresponding to the back face of the first face, the recording material should be made to pass through transfer-separation means and fixation means twice, and in this case, charge unevenness may occur in the recording material after the first passage. If a toner image is transferred to the second face with the recording material having charge unevenness in this way, the charge unevenness develops into a large noise to disturb a transfer electric field, and consequently the toner flies off, thus causing a problem such that the medium color becomes unclear, especially in the case of color images.
For achieving offset resistance during fixing, in the toner, it has been proposed that the molecular weight of the binder resin of the toner be controlled, that many types of resins be used in combination, that the viscosity be specified (see, for example, Japanese Patent Laid-Open Publication No. Hei 1-133065, Japanese Patent Laid-Open Publication No. Hei 2-161466, Japanese Patent Laid-Open Publication No. Hei 2-100059 and Japanese Patent Laid-Open Publication No. Hei 3-229265), and that properties such as a glass transition temperature, a melting temperature and a viscoelasticity be specified. In addition, Japanese Patent Publication No. Sho 52-3304 proposes various kinds of waxes such as polyethylene and polypropylene, and partially modified waxes for release agents, Japanese Patent Laid-Open Publication No. Hei 3-260659 and Japanese Patent Laid-Open Publication No. Hei 3-122660 propose that a melting point and a melting viscosity be specified, and Japanese Patent Laid-Open Publication No. Hei 7-84398 and Japanese Patent Laid-Open Publication No. Hei 6-161145 propose that properties such as a dispersion size in the toner and a toner surface exposure ratio be specified. In these propositions, however, offset resistance during fixing can be improved to some extent, but the problem of the toner flying off as a single piece during fixing cannot be solved, resulting in images far from high quality images in which letter images and line images are blurred.
In addition, Japanese Patent Laid-Open Publication No. Hei 8-305082 proposes a resin composition for toners having as a main component a vinyl based copolymer containing at least a low molecular weight polymer component with the maximum value in the molecular weight distribution being in the range of 2×103 to 4×104, and a high molecular weight polymer component with the maximum value in the molecular weight distribution being in the range of 3×105 to 8×106, and containing 0.1 to 30% by weight of calcium carbonate with the average particle size of 0.1 to 3 μm based on the total amount of resin composition. However, since this resin composition for toners contains a high molecular weight polymer component with the maximum value in the molecular weight distribution being in the range of 3×105 to 8×106, the calcium carbonate in the overall resin for toners tends to be unevenly distributed, thus making it difficult to achieve sharpness in charge characteristics, raising the possibility that charge characteristics will become inadequate.
In addition, for cleaning, cleaning characteristics are required such that a residual toner can easily drop off the surface of the photosensitive member, and the photosensitive member is not scratched when the toner is used in combination with cleaning members such as a blade and a web. In order to meet these requirements, various kinds of toners having externally added thereto inorganic fine powders such as silica, organic fine powders such as aliphatic acids, metal salts thereof and derivatives thereof, fluororesin fine powders and the like are proposed so that flowability, durability and cleaning characteristics are improved in dry developers.
In the proposed additives, however, inorganic compounds such as silica, titania and alumina can considerably improve the flowability, but tend to cause the photosensitive member surface layer to be dented and scratched due to hardness of the inorganic compound fine powder, thus raising a problem such that the toner is easily fixed in the scratched area. Furthermore, in recent years, recycled paper has been increasingly used for the sake of conservation of resources, but the recycled paper has a disadvantage that a large amount of paper powder is generated, and the paper powder or the like is trapped between the photosensitive member and the blade, thus causing cleaning defects such as black stripes. In addition, for solving these problems, a fatty acid metal salt is externally added as an additive in Japanese Patent Laid-Open Publication No. Sho 60-198556, and a wax is externally added in Japanese Patent Laid-Open Publication No. Sho 61-231562 and Japanese Patent Laid-Open Publication No. Sho 61-231563. In the methods disclosed in these patent documents, the particle sizes of additives are as large as 3 to 20 μm, and a considerable amount of additive must be added to allow the effect to be exhibited efficiently. In addition, the additive is effective in the initial stage, but cannot form a film uniformly as a lubricant due to filming unique to the additive (lubricant), thus raising a problem such that white dropouts, shading and the like occur in an image.
In addition, Japanese Patent Laid-Open Publication No. Hei 4-452 proposes titanium oxide particles treated with a fatty acid metal salt, Japanese Patent Laid-Open Publication No. Hei 5-66607 proposes titanium oxide particles with the surfaces treated with a fatty acid compound being hydrolyzed in an aqueous system, Japanese Patent Laid-Open Publication No. Hei 5-165250 proposes an inorganic compound with the surface treated with a fatty acid metal salt, and Japanese Patent Laid-Open Publication No. Hei 10-161342 proposes a fine particle titanium oxide endowed with a hydrophobic nature by treating the surface with an aliphatic aluminum. The aliphatic metal is used for the surface treatment to avoid the above described problems originating from the particle size of the fatty acid metal salt itself. In any case, however, these methods are effective to some extent, but cannot sufficiently prevent the surface of the photosensitive member from being scratched.
On the other hand, in Japanese Patent Laid-Open Publication No. Hei 2-89064, a hydrophobic hard fine powder is externally added to the toner, and the photosensitive member is chipped by means of the sanding effect of the hard fine powder to prevent toner filming. However, this method is effective in inhibition of filming, but has a disadvantage of abrading the surface of the photosensitive member, resulting in a considerable reduction in the life of the photosensitive member. At the same time, the cleaning blade is abraded with the hard fine powder, resulting in a considerable reduction in the life of the blade.
In addition, an electrophotographic toner having externally added thereto a total of 0.1 to 3 parts by weight of abrasive particles containing at least calcium carbonate and having a Mohs hardness of 3.5 or greater and a volume-based average particle size of 0.1 to 10 μm, and silica based external additive, based on 100 parts by weight of toner particles, for abrading and removing the surface oxidized layer of the photosensitive member has been proposed (see, for example, Japanese Patent Laid-Open Publication No. Hei 8-190221). In addition, in recent years, an image forming method using a photosensitive member having a hard surface has been proposed (see, for example, Japanese Patent Laid-Open Publication No. Hei 11-38656) However, if image forming is repeated using the photosensitive member having a hard surface, a problem of image flow or other image defects will arise. These image defects result from accumulation of discharge products in electrification of the photosensitive member, or deposition of the toner, paper and the like because the surface of the photosensitive member has a high level of hardness, namely the surface of the photosensitive member is hardly chipped.
The present invention has been made in view of the situation of the conventional technique described above. An advantage of the present invention is to provide an electrostatic latent image dry toner having toner flowability, charge characteristics, developing characteristics, transferability, cleaning characteristics and fixing characteristics at the same time, and capable of being satisfactorily used over a long period of time, an electrostatic latent image developer using the toner, and an image forming method.
In addition, another advantage of the present invention is to provide a toner for electrostatic latent image developer having toner flowability, charge characteristics, developing characteristics, transferability, cleaning characteristics and fixing characteristics at the same time, forming particularly the medium color of color images on both sides of the recording material when images are formed on both sides of the recording material, capable of being satisfactorily used over a long period of time, and having alleviated the problem of collecting a residual toner on a latent image holding member using an electrostatic brush, a toner for electrostatic latent image developer having the alleviated problem of collecting again a residual toner on a latent image holding member after transference in a developing device for electrostatic latent image development, and an electrostatic latent image developer using the same.
In addition, another advantage of the present invention is to provide an image forming method capable of performing development, transference and fixing to satisfy the requirement for high image quality.